A METHOD TO DETERMINE THE SHELF LIFE OF BAKE HARDENING SHEETS IN AN INDUSTRIAL PRACTICE

Abstract

The shelf life is defined as the maximum period of storage of finally processed cold rolled annealed sheets at room temperature prior to use, without showing any stretcher strain in the component after forming. Aging kinetics of cold rolled-annealed ultra low carbon bake hardening steel has been studied in the temperature range of 50- 100°C and based on these results, the activation energy for the aging process was determined as 21-24 kCal/mole, which is higher than that of diffusion of carbon or nitrogen in iron. Using this activation energy, Hundy equation was modified. The shelf life predicated using Hundy equation is significantly lower than that of actual test data, whereas the predictions made using modified Hundy equation (from higher activation energy) is close to the actual room temperature aging data. In the present invention a simple method has been developed to determine the shelf life of ultra low carbon steel in an industrial practice.

Full Text

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FIELD OF THE INVENTION
The Invention relates to the development of a method to determine
the shelf life of bake hardening steel in industrial practice.
BACKGROUND OF THE INVENTION
The steel for automobile body panel applications requires higher
strength for better dent resistance and good formabillty for defect
free formation of the component of automobile. In view of the
above, bake hardening steel was developed having a good
combination of yield strength and formabillty and it gives rise to an
appreciable increase in yield strength of 30-50 MPa during
commercial baking operation (around 17 °C / 20min) of the formed
component This extra increase in strength leads to an improvement
in dent resistance. A thinner gauge of this material can also be used
replacing the conventional thicker material. In advanced automobile,
use of bake hardening steels is well established due to the additional
strength achieved during the commercial baking operation. This
bake hardening is primarily achieved by the presence of controlled
amount of interstitial carbon in solid solution after cold roiling and
annealing. Ultra low carbon (ULC) steel is the prime material for
bake hardening (BH) grade, since it possesses excellent formability
and it is easier to control the solute carbon to a desired level at the

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steel making stage. In addition, these steels can easily be processed
through various routes such as batch annealing, hot dip galvanizing
gaivannealing route and continuous annealing route. However,
automakers are concerned about the room temperature aging
problem in bake hardening steels due to inappropriate amount of
solute elements, which may cause stretcher strain during forming at
automobile manufacturer's place. This leads to a rejection of
material adding to an extra cost for manufacturing. The requirement
is that the material should have sufficient shelf life so that it should
not degrade during the transportation and storage before the forming
operation. The knowledge of the shelf life is necessary for both steel
makers and automakers. For steel makers, it is important to know
the shelf life in order to control the inventory and decide a period for
its storage, transportation and its delivery, whereas for automobile
component manufacturers it is useful in deciding its storage period
and controlling inventory to produce the automobile components
without any rejection. There is no standard available to evaluate the
shelf life of bake hardening steel and hence different automakers use
their own technique for its evaluation. Some of the them are using
the conventional method i.e. strain aging, which is mainly based on
the rise in the flow stress after accelerated aging test at 100 °C after
7.5-10% tensile deformation. The method does not suit for
predicting the room temperature aging behaviour of ultra low carbon
steel in skin-passed condition, specially to be used for manufacturing
auto-components, where stretcher strain caused by yield point

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elongation of the input material is the main concern. Hundy has
presented an expression for simulating the aging behaviour at
different temperature based on Cottrell-Bilby model an the prediction
made by it deviates from the actual room temperature test data in
case of skin-passed or temper rolled material. Carrying out actual
room temperature test for shelf life evaluation is not possible in view
of long time and large number of samples required for this. Thus
there is a need to formulate a simple method for the prediction of
shelf life in an industrial practice. In view of the limited work in this
area, the present invention focuses on evaluating the strain aging
behaviour of ultra low carbon bake hardening steel and then
formulating a simple method to determine the shelf life applicable In
an industrial practice.
OBJECTS OF THE INVENTION
- the object of the invention Is to determine the shelf life of bake
hardening steel in an industrial practice.
- another object of the invention is to reduce the rejection loss of
bake hardening steel for steel manufacturers and ancillary users.
- further object of the Invention is to optimise production /
inventory control of steel manufacturers and ancillary users.

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- yet another object of the invention is to formulate a simple
method for ascertainment of shelf life of bake hardening steel.
DETAILED DESCRIPTIONS Of THE PREFERRED EMBODIMENT
Cold rolled ultra low carbon (ULC) steels alloyed with Mn and P were
taken for present investigations. The bake hardening strength in
these steels were found to vary from 31 to 43 MPa. For assessing
the kinetics of aging, the tensile specimens were aged for various
periods at different aging temperatures (Ta) i.e. 50, 70, 85 and 100
°C. The yield point elongation (YP-EL) was then measured by
conducting tensile test. To study the room temperature aging
behaviour, the samples were aged for various periods in oil bath
furnace maintained at 30 °C and then yield point elongation were
similarly measured.
For determination of the bake hardening strength, tensile specimens
were pre-strained to 2%, and then heated at 170 °C for 20 minutes
in an oil bath furnace. The aged sample was again tested on a
tensile machine to measure lower yield stress. BH strength was
evaluated as the difference between lower yield stress after baking
and flow stress after 2% elongation.

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Kinetics of aging process was assessed using following equation,
which is derived form of Johnson - Menl - Avrami (3 - M - A)
equation:
In (ty) = C + Q / RT — (1)
Where ty = time for a given fraction of transformation, and Q =
Activation energy for the governing process. In the present analysis,
the time for yield point appearance was used as ty. The In (ty) was
plotted for different steels as a function of 1 / T and the activation
energy calculated from the slope for the different steels was found to
be in the range of 21 - 24 kCal / mole.
Hundy derived the following correlation using Cottrell-Bilby equation
log [t/t] = K [l/Tr-l/T] - log [T/Tr] — (2)
Where, K = constant = Q / R (log10e), Q = Activation energy, R =
gas constant, (for carbon, K = 4400; for nitrogen, K = 4000), Tr =
room temperature (K), T = artificial aging temperature (K), t = aging
time at room temperature, Tr t= aging time at artificial aging
temperature T. It enables the aging time at any temperature to be
converted to an equivalent aging time at room temperatures. The
constant K was derived from the activation energy for the diffusion of
C or N in iron. The above equation is reported to deviate from the
actual room temperature test data in temper rolling condition. This

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could be due to the higher activation energy in the skin passed or
temper rolled material. The average activation energy determined in
the present results was used to re-calculate the exact K-value of the
Hundy equation and thus modified Hundy equation can be
represented as follows:
Log [tr/tr] = 5030 [1/Tr-1/T] - Log [T/Tr ]- (3)
The shelf life was predicted by both Hundy equation and modified
Hundy equation. The pedictions made by Hundy equations are much
lower than that of actual room temperature test data, whereas the
prediction made using modified equation matches well with them. In
the present work, the shelf life was defined as the time of room
temperature aging when the yield point elongation becomes 0.2%.
This level of point elongation may be acceptable by the automobile
manufacturers. To determine the exact shelf life (time for YPEL-
0.2%), it is necessary to conduct a large number of tests at a high
temperature for simulation. In the present invention a simple
method has been designed for evaluating the shelf life in an
industrial practice, YP-EL value at the aging temperature of 100 °C
for lh was selected as the aging parameter for accelerated ageing
test. A relationship between the shelf life as a function of YP-EL for
100 °C/lh has been established as shown in fig. 2. The laid down
criteria is shown in table-III. Based on this relationship, the

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expected shelf life can be assessed by a single simple test and can be
utilized in industrial condition as a routine basis. The selecting "100
°C" as the test temperature is important since this temperature can
be achieved accurately in a boiling water bath and it can be used in
place of using furnace.
A typical graph for the experimental evaluation of shelf life for such
sample steel is shown in Fig 1.
The stretcher strain in the formed component Is related to the yield
point elongation of the material before forming. It has been
observed that there is no occurrence of stretcher strain up to a
certain YPEL value. A limit of 0.2% YPEL for no stretcher strain has
been considered.
One can decide any test temperature or aging time for simulation. In
the present method, aging parameter as 100 °C for lh has been
selected for the convenience of the test in industrial condition. Test
temperature as 100 °C has been selected since this is the boiling
point of water. The samples can be heat-treated in boiling water
easily without any variation in temperature. An appropriate time of
lh has been decided since too long time period may not be possible
for conducting this test on routine basis in an industrial condition.
Shorter time period may result in a very low yield point elongation
causing an error in measurement.

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Similar aging parameter (100 °C/lh) has also been suggested in a
paper reported by Hundy for simulation for aging process.
The relationship has been established and shown In Fig 2. And shelf
life can be ascertained from the chart as shown in Table - III from
above experiments.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
1. Fig - 1 shows an experimental evaluation of shelf life and its
validation with the actual room temperature test (assumed as 30
°C).
2. Fig - 2 shows relationship between the shelf life and yield point
elongation.
3. Fig - 3, shows relationship between Aging time and Yleldpolnt
Elongation
4. Fig - 4, shows slopes of various steel.
5. Table - III shows a simulated relationship data of YPEL at 100 °C
/ lh (%) and shelf life.

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SUMMARY OF THE INVENTION
The shelf life is defined as the maximum period of storage of finally
processed cold rolled annealed sheets at room temperature prior to
use, without showing any stretcher strain in the component after
forming.
Aging kinetics of cold rolled-annealed ultra low carbon bake
hardening steel has been studied in the temperature range of 50-
100°C and based on these results, the activation energy for the aging
process was determined as 21-24 kCal/mole, which is higher than
that of diffusion of carbon or nitrogen in iron. Using this activation
energy, Hundy equation was modified. The shelf life predicated
using Hundy equation is significantly lower than that of actual test
data, whereas the predictions made using modified Hundy equation
(from higher activation energy) is close to the actual room
temperature aging data. In the present invention a simple method
has been developed to determine the shelf life of ultra low carbon
steel in an industrial practice.

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WE CLAIM
1) A method to determine the shelf life of bake hardening steel In
industrial practice.
2) The method as claimed under claim 1 characterised in that
preparation of test data of yield point elongation at different
temperatures and time for samples of cold rolled ultra low carbon
steels (ULC) alloyed with Mn and P.
3) The claim as claimed under claim 1 and 2 characterised by pre
straining of the said samples to 2% and heating at 170 °C for 20
minutes, and evaluation of bake hardening strength as difference
between lower yield stress after baking and flow stress after 2%
elongation.
4) The claim as claimed under claim 1, 2 and 3 characterised in that
determination of activation energy from the test data and slope of
various steels (In (ty) VSV1/T) as shown in fig - 4.

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5) The claims as claimed under claims 1, 2, 3 and 4 characterised in
that simulation of strain aging and determination of shelf life
through plotting the graphs as shown in Fig - 1 and Fig - 2 from
the derived test data.

The shelf life is defined as the maximum period of storage of finally
processed cold rolled annealed sheets at room temperature prior to
use, without showing any stretcher strain in the component after
forming.
Aging kinetics of cold rolled-annealed ultra low carbon bake
hardening steel has been studied in the temperature range of 50-
100°C and based on these results, the activation energy for the aging
process was determined as 21-24 kCal/mole, which is higher than
that of diffusion of carbon or nitrogen in iron. Using this activation
energy, Hundy equation was modified. The shelf life predicated
using Hundy equation is significantly lower than that of actual test
data, whereas the predictions made using modified Hundy equation
(from higher activation energy) is close to the actual room
temperature aging data. In the present invention a simple method
has been developed to determine the shelf life of ultra low carbon
steel in an industrial practice.The shelf life is defined as the maximum period of storage of finally
processed cold rolled annealed sheets at room temperature prior to
use, without showing any stretcher strain in the component after
forming.
Aging kinetics of cold rolled-annealed ultra low carbon bake
hardening steel has been studied in the temperature range of 50-
100°C and based on these results, the activation energy for the aging
process was determined as 21-24 kCal/mole, which is higher than
that of diffusion of carbon or nitrogen in iron. Using this activation
energy, Hundy equation was modified. The shelf life predicated
using Hundy equation is significantly lower than that of actual test
data, whereas the predictions made using modified Hundy equation
(from higher activation energy) is close to the actual room
temperature aging data. In the present invention a simple method
has been developed to determine the shelf life of ultra low carbon
steel in an industrial practice.